Good advice above.
Additionally, let me add a little from personal experience. On my third month at Headley Court I was told my injury (leg / accident) would get no better, and that there was a possibility of a medical discharge with medical pension. The inference was that the swing of the pendulum, so to speak, could also be influenced by my preference of choice.
On asking what the pension might be, a fair question I thought, having a family to consider too, I was told that no figure could possibly be given until after a med board. Needless to say, I found this too much of a risk, even if it was to be tax free, and did everything in my power to stay in and make 22 years.
Furtunately, I achieved this, even getting a royal bollocking on my med board for having passed my fitness test (40+ on bike) whilst exempt!
Hope that helps, whatever way you elect to try for (assuming you have a choice), think carefully, and tread even more so.
Oh, you will also likely qualify for a War Pension, and that too is tax free, and depending on severity of injury, normally a lump sump and monthly payment. Also check the AP, the rules changed whilst I was perm, downgraded, and promotions were allowed, but no extension of service. That played in my favour and I came out on Flt Sgts pension, so all in all, not too bad in the long run.
Good luck & best wishes.